There probably isn't enough traffic on a dual carriageway at 6am, to worry about. If you are hit by a car at 60-70mph then your head is probably the last thing to worry about esp as helmets only offer protection up to 12mph.
Motorists on a dual carriageway are usually aware that there is an event on due to the high number of cyclists with reflective numbers and the signage and presence of marshalls. To be honest I find cycling on "quiet" lanes more daunting sometimes, too many idiots trying to avoid the law cos it's quieter, uninsured drivers on the wrong side of the road..........
In 30 years I have had one car hit me in a time trial and 3 hit me on "so called" quiet roads, all of those resulted in the worst injuries.
At the end of the day it is people's choice and that is something the CTC vigorously supports, that and a whole host of other cyclists' rights. If you want to see lots of discussions for and against helmets and whole host of other issues check out the ctc forum. 